jaycar/silicon chip troubles!

I recently purchased a jaycar/silicon chip theremin kit.

Having spent £65/ £10 delivery and a further £10 on a power supply, you can understand my frustration at this moment!

I have assembled the everything as per instructions, being as careful as possible with the soldering. now that it is finished i have these problems:

I can get a constant hum out of the unit, but after about 15 seconds, the hum reduces in intensity - i cannot change the hum with either of the aerials.

Secondly, two components heat up to the point you cannot touch them within 15-20 second of powering on.
The components in question are: top right of the circuit board labeled "7805" - looks like a black square with a thin piece of metal on one side extending up with a hole in it.

the second is: one of the smaller microchip things labeled "LM358".

Any suggestions?

Thanks in advance!
Dave

Sounds a whole lot like you reversed the + and - leads of the power supply. The + lead needs go to the "input" side of the 7805. The minus lead connects to circuit board ground.

Less likely, but also possible is that you installed the 7805 or the LM386 chip backwards.

Also look at the underside of the board very carefully to check for solder splashes between IC pins or between traces.

Don

wow

such a simple thing! reversed the chip, and the pitch changed on moving closer to the aerial =D

BUT

any suggestions on how to tune the damn thing!
Dave

... also make sure you didn't put the LM358 in the wrong socket!

If the circuit is like the 2009 version presented on TW today, there is a LM386 power amp chip and an unidentified opamp ... probably your LM358 is that opamp, and it will definitely not like sitting in the LM386's circuit. If that is what you did, that LM358 is history.

Check polarity of electrolytic capacitors too. If the one across the input of the regulator is backwards, it will draw a lot of current.

Don

There is no way (that I can see) to tune this instrument other than adjusting the core of T1. See my posting in “ Topic: New Silicon Chip Theremin 2009 edition (http://www.thereminworld.com/forum.asp?F=3&T=3873&cmd=p)”

The tuning procedure should be in the instructions under setting up?

If not, some of the modification articles include it.

Regards

MarkT

I am happy that you got it working ... hope some of my suggestions helped in troubleshooting it.

Another helpful hint, regarding tuning ... go get yourself the correct size plastic tuning tool (if the transformers have the hex-head ferrite rod) or a plastic screwdriver. That will minimize the amount of interaction between you and the transformers while tuning.

Also try to adjust in as small of increments (1/4 turn, 1/2 turn, single turn) as you can, and only tune one coil at a time if you can. This is based on Fred's assumption that this design won't have as much range of tuning as the previous one.

If you have a frequency counter or 'scope, you can observe the oscillator frequencies and will be able to set the local (fixed oscillator) higher than the variable (pitch oscillator). That will probably get you "in the ballpark".

If the Volume section is not working, you might try lifting the side of the 6.8k resistor that connects to TP3 (why the heck don't they have any reference designators on that schematic!!). Connect a fixed voltage from that free end of the 6.8k to GND. A 0-5V DC power supply would be useful for this. You want to set the voltage so that the 1496 chip has maximum gain. You should be able to get signal out to the line output then.

If that is the case, then you know that the problem lies in the Volume Osc. circuit (the stuff at the bottom of the page).

Don